Plant transport tissues - xylem and phloem

Plants have two transport systems - xylem and phloem. Xylem transports water and minerals. Phloem transports sugars and amino acids dissolved in water.

Xylem

The xylem transports water and minerals from the roots up the plant stem and into the leaves.

In a mature flowering plant or tree, most of the cells that make up the xylem are specialised cells called vessels. Vessels:

  • Lose their end walls so the xylem forms a continuous, hollow tube. This allows water to flow easily.
  • Become strengthened by a chemical called lignin. The cells are no longer alive. Lignin gives strength and support to the vessel.

Transport in the xylem is a physical process. It does not require energy.

Diagram showing how the xylem transports water to the rest of the plant

Phloem

Phloem moves sugar that the plant has produced by photosynthesis to where it is needed for processes such as:

  • growing parts of the plant for immediate use
  • storage organs such as bulbs and tubers
  • developing seeds
  • respiration

Transport in the phloem is therefore both up and down the stem. Transport of substances in the phloem is called translocation.

Phloem consists of living cells. The cells that make up the phloem are adapted to their function:

  • Sieve tubes - specialised for transport and have no nuclei. Each sieve tube has a perforated end so its cytoplasm connects one cell to the next.
  • Companion cells - transport of substances in the phloem requires energy. One or more companion cells attached to each sieve tube provide this energy. A sieve tube is completely dependent on its companion cell(s).
Diagram showing how the phloem moves food substances around the plant

Comparison of transport in the xylem and phloem

XylemPhloem
Type of transport Physical processRequires energy
Substances transportedWater and mineralsProducts of photosynthesis, including sugars and amino acids dissolved in water
Direction of transportUpwardsUpwards and downwards

Plant organisation

The xylem and phloem are distributed differently in roots and stems. In the root, the xylem forms a central column, forming a solid support. The phloem is towards the centre, outside the xylem.

In the stem, the transport tissues of the xylem and phloem are grouped into vascular bundles.

Cross section of a plant root and plant stem